During various technological processes, among others also during the coking of stone coal, waste waters result which are contaminated with hydrocarbons. Primarily, the polycyclic hydrocarbons (PAH) possibly contained in the waste water, have to be removed to the largest possible extent. Especially biological waste water stages are additionally loaded with a high concentration of hydrocarbons.
Other processes of waste-water purification, for instance reverse osmosis or adsorptive treatment, require a preliminary purification stage.
Depending on their concentration or chemical composition, the hydrocarbons contained in the waste water are partially dissolved and particularly bound to particles existing in the waste waters.
For this reason it is desirable to have a purification process which removes simultaneously not only the water-soluble hydrocarbons, but also the ones bound to solid substances.
For the separation of hydrocarbons from waste waters of coking plants, it has already been proposed to obtain an at least partially effective separation by filtration through a filter aid of wood dust in a vacuum revolving filter (Fachberichte Huttenpraxis Metallverarbeitung, Vol. 22, No. 10, 1984, 1088-1092). This filter aid is quickly clogged by the entering fine particles, so that an external, thin filtering layer has constantly to be removed by a scraper. Especially the use of wood dust for the filter layer in connection with the use of a vacuum revolving filter, due to rapid transitions from soaking to drying of the particles resulting from the revolving of the drum, lead to so-called drying cracks, and thus to the escape of suspension. Very thick filter cake layers facilitate drying cracks.
The use of wood dust as filter aid impairs considerably the economy of the process, since it often represents a substance which is external to the operation.
Also, by the use of vacuum for the drying of the filter cake, considerable costs are incurred.
Besides wood dust, other commercially available filter aids are used, such as kieselguhr, celit, or cellulose. Primarily due to cost reasons or to additional problems connected to the disposal of the filter residue, the use of such filter aids is made difficult.
For a clean adsorptive waste water purification, it is possible to use activated carbons or cokes (VDI Forschungsheft No. 607/1981). This process, however, requires a high technical effort for the regeneration of adsorbents since especially for costs reasons the disposal of loaded activated carbon through other methods (burning or storage) is prohibitive. Besides the waste water in adsorptive purification has to be extensively free of solid substances. The solid substances often found in waste waters are thus extremely damaging to the adsorptive waste-water purification. These solid substances accumulate in the adsorbent and cannot be completely removed even during the regeneration of the activated coal. As a result, the efficiency of the adsorbent decreases. A removal of the solid foreign substances has to take place separately from the adsorptive purification stage.
A further process for the purification of waste waters is the reverse osmosis. This purification process makes it possible for the operator to obtain a permeate almost free of noxious substances by concentrating the noxious substances present in the solution. Damaging are in this case the insoluble and colloidal impurities which block (clog) the reverse-osmosis membrane. A relative measure for the clogging effect of suspensions is the blocking index. The method for the determination of the blocking index is described in the ASTM-regulation D 4189-82 "Silt Density Index of Water". Tests have proven that when the blocking index is lower than three, a clogging of the membranes is not to be expected in the operation of reverse-osmosis installations. The preliminary purification of the suspension for the reduction of the blocking index, which is necessary for the reverse osmosis, can be performed for instance by using one-way filter cartridges of flush-back filters.